Because (1) a tank full of petrol can easily be 85% petrol by weight, (2) when it burns in your ICE, over 75% of the resulting weight (CO2 and H2O) is "free for the taking, everywhere you go" atmospheric O2, and (3) you need to keep carrying 0% of that exhaust weight with you.
With batteries, all those percentage go to crap. Plus you've got the extra structure needed to carry all the extra weight, all the time.
A new article exploring why electric vehicles are heavier than gasoline-powered vehicles, why automakers cannot simply “use smaller batteries,” and whether improvements in battery design can make EVs lighter. It also looks at the tradeoff between battery mass and driving range, and foreshadows the implications of EVs becoming a larger share of the vehicle fleet in the US and around the world.